1. Field of the Invention
This invention relates to a bearing structure of a rotary machine such as a turbo-charger, an energy recovery apparatus, and the like.
2. Description of the Prior Art
An exhaust dynamo apparaus consisting of an exhaust turbine driven by exhaust energy of an internal combustion engine and a dynamo driven by the exhaust turbine is heretofore known from Japanese Patent Laid-Open No. 95124/1985, for example. This exhaust dynamo apparatus comprises an exhaust turbine actuated by exhaust energy of an internal combustion engine and a dynamo driven by the exhaust turbine, and a turbine blade shaft of the exhaust turbine and a rotor shaft of the dynamo are connected on the same axis. The turbine blade shaft and the rotor shaft are made of a ceramic material and the rotor shaft is supported on housings through a fixed bearing, an oil float bearing and a thrust bearing. The bearing for supporting rotatably the rotor shaft employ fluid lubrication.
Generally, the shaft to which the exhaust turbine operated by exhaust energy of an internal combustion engine is fitted rotates at an extremely high speed. The rotating speed of the shaft of a turbo-charger or energy recovery apparatus equipped with the exhaust turbine, for example, is as high as 100,000 to 150,000 r.p.m. The turbine blade itself of the exhaust turbine is exposed to the high temperature exhaust gas and the shaft connected to the turbine blade shaft also attains a high temperature state. The bearing portion for supporting the shaft of such a ultra-high speed rotary machine relies more heavily on fluid lubrication by a liquid lubricant such as oil than on boundary lubrication which depends on a friction coefficient. However, even in a rotary member using fluid lubrication, its friction is great and a considerable quantity of oil must be fed into the bearing in the case of components such as the turbo-charger which are exposed to the high temperature. In the case of fluid lubrication using oil, quite a complicated oil seal having high sealing property must be used in order to prevent the leak of the oil, and various functional components such as oil pipes, oil drains, and the like, are necessary. In the ultra-high speed rotary machine such as the turbo-charger or the energy recovery apparatus, however, the place at which the rotary shaft is to be supported on the housings is positioned at an extremely inconvenient position.
The problems described above can be solved if ball bearings made of a ceramic material can be used for this ultra-high speed rotary machine. Even if the ball bearing is made of a ceramic material, particularly silicon nitride, friction becomes remarkable if no lubricant at all exists, because it supports the rotary shaft which rotates at an extremely high rotating speed.
Incidentally, the lubrication characteristics of solid lubrication by solid lubricants such as CaF.sub.2, Cr.sub.2 O.sub.3, etc, and liquid lubrication by liquid lubricants such as oil are affected greatly by temperatures and large differences exist between them. As shown in FIG. 2, when the temperature T is low, the friction coefficient .mu. is great for solid lubrication but when the temperature T rises, the friction coefficient .mu. drops. With liquid lubrication, on the other hand, the friction coefficient .mu. is low when the temperature is low but as the temperature T rises, the friction coefficient .mu. increases drastically. For instance, the friction coefficient .mu. is 0.2 at 300.degree. C. for liquid lubrication but the friction coefficient .mu. for solid lubrication is about twice that of liquid lubrication at 300.degree. C. When the temperature is 600.degree. C., however, the friction coefficient .mu. is 0.2 for solid lubrication but it becomes several times for liquid lubrication.